The present invention is directed toward the field of battery protection systems. In particular, the invention is directed to a battery disconnect system for disconnecting a battery from its load during long periods of inactivity to reduce the likelihood that the battery will become damaged due to excessive battery discharge.
Many batteries such as rechargeable batteries may be irreparably damaged if they are discharged too deeply. Lithium-based batteries are especially susceptible to this hazard.
Many devices having built-in rechargeable batteries can expose their rechargeable battery to this excessive discharge hazard. Even when not in use, the rechargeable battery remains connected to the device's load. The small load the device presents to the rechargeable battery during inactive periods will cause the rechargeable battery to slowly discharge, potentially damaging the battery.
This problem is amplified when devices having rechargeable batteries are transferred from a manufacturer to a retailer. Prior to being sold to an end-user, the device may be warehoused or placed on shelves for weeks or months before the end-user first uses the device. The period of time that the device remains in the warehouse or on the retailer's shelf may be sufficient to allow the battery to become over-discharged and damaged. To alleviate this problem, it is desirable to have a method of preventing the device from over-discharging during periods of long inactivity.
One method of addressing this problem has been for manufacturers to position plastic tabs between the battery and the power terminal to physically disconnect the load from the battery. Upon purchase of the device, the customer is then instructed to remove the tab thereby reconnecting the load to the device.
This method has many disadvantages. One disadvantage is that it poses significant impediments to the manufacturer's quality assurance program. During a quality assurance inspection, the tab must be removed to allow the inspector to test whether the product functions, and then must be reinstalled in the product, which may be extremely difficult. This method also can reduce the product's overall reliability due to the addition of mechanical controls. Another shortcoming is that the end-user has to ensure that the rechargeable battery is properly installed and connected after removal of the tab.
Other proposed methods include adding mechanical switches that can add significant resistance to the battery path thereby degrading performance. Also, the mechanical nature of the switches often results in a reduction of overall product reliability.
Another method of addressing the problem has been the use of commercially available Lithium battery pack protection circuits. These battery pack protection circuits measure the lithium cell voltage and disconnect the battery from the load if the battery voltage drops below 2.5 volts or other predetermined thresholds. One disadvantage of this method is that several hours are required for the battery to be charged back to a useable state after the battery pack protection circuit has disconnected the battery from the load--an unacceptable result for electronic devices that are designed to function with little, if any perceived downtime. Another disadvantage is that disconnecting the load after the battery voltage has dropped to a pre-determined level such as 2.5 volts does not guarantee that the battery will not be damaged. The battery will continue to discharge as a result of self-discharge and power drain from the protection circuit. Lithium cells become damaged if they are discharged below a low voltage level (typically 2.5 volts) for more than a few minutes. Consequently, the battery may still over-discharge when utilizing a load-disconnecting method that does not compensate for self-discharge and power drain and thereby does not protect against damage to the lithium cell.
Therefore, there remains a need in this art for a battery protection system that can disconnect the load from the battery to prevent the battery from over-discharging without the use of a removable mechanical interlock device. There remains a further need for a system that can disconnect the load from the battery before the battery has been significantly discharged.